Radiation-sensitive silver chloride photographic emulsions are known to offer specific advantages. For example, silver chloride exhibits less native sensitivity to the visible portion of the spectrum than other photographically useful silver halides. Further, silver chloride is more soluble than other photographically useful silver halides, thereby permitting development and fixing to be achieved in shorter times.
It is well recognized in the art that silver chloride strongly favors the formation of crystals having {100} crystal faces. In the overwhelming majority of photographic emulsions silver chloride crystals when present are in the form of cubic grains. With some difficulty it has been possible to modify the crystal habit of silver chloride. Claes et al, "Crystal Habit Modification of AgCl by Impurities Determining the Solvation", The Journal of Photographic Science, Vol. 21, pp. 39-50, 1973, teaches the formation of silver chloride crystals with {110} and {111} faces through the use of various grain growth modifiers. Wyrsch, "Sulfur Sensitization of Monosized Silver Chloride Emulsions with {111}, {110} and {100} Crystal Habit", Paper III-13, International Congress of Photographic Science, pp. 122-124, 1978, discloses a triple-jet precipitation process in which silver chloride is precipitated in the presence of ammonia and small amounts of divalent cadmium ions. In the presence of cadmium ions control of pAg and pH resulted in the formation of rhombododecahedral {110}, octahedral {111}, and cubic {100} crystal habits.
Tabular silver bromide grains have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The term "substantially parallel" as used herein is intended to include surfaces tha appear parallel on direct or indirect visual inspection at 10,000 times magnification. The aspect ratio--that is, the ratio of diameter to thickness--of tabular grains is substantially greater than 1:1. High aspect ratio tabular grain silver bromide emulsions were reported by deCugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125.
From 1937 until the 1950's the Eastman Kodak Company sold a Duplitized.RTM. radiographic film product under the name No-Screen X-Ray Code 5133. The product contained as coatings on opposite major faces of a film support sulfur sensitized silver bromide emulsions. Since the emulsions were intended to be exposed by X-radiation, they were not spectrally sensitized. The tabular grains had an average aspect ratio in the range of from about 5 to 7:1. The tabular grains accounted for greater than 50% of the projected area while nontabular grains accounted for greater than 25% of the projected area. The emulsion having the highest average aspect ratio, chosen from several remakes, had an average tabular grain diameter of 2.5 microns, an average tabular grain thickness of 0.36 micron, and an average aspect ratio of 7:1. In other remakes the emulsions contained thicker, smaller diameter tabular grains which were of lower average aspect ratio.
Although tabular grain silver bromoiodide emulsions are known in the art, none exhibit a high average aspect ratio. A discussion of tabular silver bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photograhic Emulsions," Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
It has been recognized that advantages in covering power and other photographic characteristics can be obtained by preparing silver halide emulsions in which the grains are tabular--that is, areally extended in two dimensions as compared to their thickness. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounded by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1 by a double-jet precipitation technique in which pAg is controlled within the range of from 5.0 to 7.0. As shown in FIG. 3 of Bogg, the silver halide grains formed exhibit square and rectangular major surfaces characteristic of {100} crystal faces. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of monosize silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, causing the seed crystals to increase the size by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Lewis does not mention silver chloride. Maternaghan U.S. Pat. Nos. 4,150,994 and 4,184,877, U.K. Pat. No. 1,570,581, and German OLS publications, 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromoiodide containing 40 mole percent iodide also contains 60 mole percent bromide.) Japanese Pat. No. Kokai No. 142,329, published Nov. 6, 1980, appears to be essentially cumulative with Maternaghan, but is not restricted to the use of silver iodide seed grains.
Perignon U.S. Pat. No. 3,784,381 teaches the preparation of silver chloroiodide and silver chlorobromoiodide emulsions by precipitating the silver halide grains at a pH in the range of from 5 to 9 and a pAg of at least about 7.8 by adding to the precipitation mixture no later than at the end of the precipitation a weak solvent for silver halide selected from the group consisting of ammonium chloride, ammonium nitrate, and magnesium chloride.
Wilgus and Haefner U.S. Ser. No. 429,420, filed concurrently herewith and commonly assigned, titled HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, discloses high aspect ratio silver bromoiodide emulsions and a process for their preparation.
Kofron et al U.S. Ser. No. 429,407, filed concurrently herewith and commonly assigned, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE EMULSIONS AND PHOTOGRAPHIC ELEMENTS, which is a continuation-in-part of U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions. Kofron et al discloses that photographic elements can exhibit improved sharpness and exhibit a wider separation between their speed in a spectral region of intrinsic sensitivity and a spectrally sensitized region. Kofron et al further discloses speed-granularity advantages for high aspect ratio tabular grain silver bromoiodide emulsions.
Daubendiek and Strong U.S. Ser. No. 429,587, filed concurrently herewith and commonly assigned, titled AN IMPROVED PROCESS FOR THE PREPARATION OF HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS, which is a continuation-in-part of U.S. Ser. No. 320,906, filed Nov. 12, 1981, now abandoned, discloses an improvement on the processes of Maternaghan whereby high aspect ratio tabular grain silver bromoiodide emulsions can be prepared.
Abbott and Jones U.S. Ser. No. 430,222, filed concurrently herewith and commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED CROSSOVER, which is a continuation-in-part of U.S. Ser. No. 320,907, filed Nov. 12, 1981, now abandoned, discloses the use of spectrally sensitized high aspect ratio tabular grain emulsions in radiographic elements coated on both major surfaces of a radiation transmitting support to control crossover. Comparisons of radiographic elements containing high aspect ratio tabular grain emulsions with similar radiographic elements containing conventional emulsions show that reduced crossover can be attributed to the high aspect ratio tabular grain emulsions. Alternatively, comparable crossover levels can be achieved with reduced silver coverages and/or improved speed-granularity relationships.